1. Field of the Invention
The present invention relates to a data retransmitting method, and more particularly to a data retransmitting method which performs error recovery when a transmission error has developed in case of the transmission of data through a communication satellite.
2. Description of the Prior Art
In the case of using a channel, the propagation delay time of which is as great as 250 msec. each way, as in satellite communication, the transmission efficiency lowers drastically when employing a procedure in which the next block of data cannot be sent from the transmitting station until the reception acknowledgement response of the last transferred data block arrives from the receiving station. In satellite communication, therefore, a method capable of continuous transfer has been adopted and the HDLC (High level Data Link Control) protocol stipulated by the ISO (International Organization for Standardization) has been used.
In the HDLC protocol, the REJ (Reject) scheme and the SREJ (Selective Reject) scheme are stipulated as retransmission schemes to be used in the case where a transmission frame (a transmission data block to which a header for control is added) has undergone an error and has not properly reached the opposite side.
The REJ scheme is representative of the so-called "Go-Back-N Automatic Repeat Request scheme." As illustrated in FIG. 1 by way of example, in a case where the receiving side detects the omission of Frame No. 5 because of the reception of Frame No. 6 subsequent to Frame No. 4 and sends back a signal REJ5 indicating a retransmission request for the frame No. 5, the sending side retransmits the erroneous frame (No. 5 frame) and also a series of sent frames (No. 6-No. 15 frames) succeeding thereto (the retransmission is indicated by double lines). However, the satellite channel exhibits a great propagation delay time as stated before, and the number of frames to be transmitted wastefully increases as the speed of transmission becomes higher. More specifically, as illustrated in FIG. 2, when data is passed through the satellite channel, the propagation delay time of 250 msec.+.alpha. is taken one way, and the propagation delay time of 500 msec.+2.alpha. is taken in a reciprocation, during which about 750 frames can be sent. Accordingly, in a case where a negative or rejection response indicating the development of a transmission error in Frame No. k is sent back to the sending side immediately after the sending side has sent Frame No. (n+1), a delay time of 500 msec.+2.alpha.+(n+1-k).beta. is additionally required for the retransmission of Frames No. k thru No. (n+1). Here, .beta. denotes the transmission time of one frame. (Reference literature: D. W. Andrews, "Throughput efficiency of logical links over Satellite Channels," International Symposium on Satellite and Computer Communications, Apr. 19, 1983, p. 231-242).
In the case of FIG. 2 where the speed of the satellite channel is 1.544 Mb/sec., when the erroneous frame has arisen, about 750 frames each having a length of 125 bytes are wastefully transferred. On the other hand, this REJ scheme has the advantage that a small number of buffers suffice on both the sending and receiving sides.
Meanwhile, according to the SREJ scheme, only erroneous frames are retransmitted as illustrated in FIG. 3. Therefore, no frame is transferred wastefully. Since, however, two or more signals SREJ cannot be simultaneously sent back according to the provisions of the HDLC protocol, large numbers of buffers are required on both the sending and receiving sides. At present, only the REJ scheme is used, and the SREJ scheme is merely stipulated, but is not generally used.